An accelerometer is typically an electromechanical system. A single-axis or one dimensional accelerometer includes an acceleration sensor, i.e. mechanical transducer, and an electronic signal detection, conditioning and control circuit to convert a mechanical signal to an electronic signal. The acceleration sensor comprises a proof mass suspended by a spring or multiple springs on a support structure with electrodes or other means to detect proof mass displacement. A force or acceleration applied on the structure results in a displacement of proof mass from a reference position and the displacement is detected by an electronic sensing circuit which produces analog or digital output signals proportional to the acceleration.
To satisfy performance requirements of dynamic range, linearity, bandwidth, as well as high-sensitivity and low-noise, a high-performance accelerometer is often designed to operate in a closed-loop feedback system as a force-rebalanced accelerometer. In a force-rebalanced accelerometer, a forward circuit detects and generates a signal which is representative of the displacement that the mass has moved from a reference position due to acceleration. And a feedback circuit, in responding to the output signal of the forward circuit, generates a restoring force to return the mass to the reference position.
Accelerometers can be designed and implemented on a silicon material in form of micro-electromechanical systems (MEMS). Micromachined force-rebalanced accelerometers with capacitive sensing and electrostatic forcing have demonstrated advantages over other types of accelerometers in a combination of high-sensitivity, low-noise, good direct current (DC) response and wide bandwidth. Accelerometers of this type are shown in prior arts such as U.S. Pat. No. 4,922,756 to Henrion in 1990, U.S. Pat. No. 4,930,043 to Wiegand in 1990, U.S. Pat. No. 5,095,752 to Suzuki et al in 1992, U.S. Pat. No. 5,345,824 to Sherman et al in 1994, U.S. Pat. No. 5,616,844 to Suzuki et al in 1997, U.S. Pat. No. 5,652,384 to Henrion et al. in 1997, U.S. Pat. No. 5,852,242 to Devolk et al. in 1998, U.S. Pat. No. 6,035,694 to Dupuie et al. in 2000, U.S. Pat. No. 6,805,008 to Selvakumar et al. in 2004.
FIG. 1 shows a block diagram of an analog feedback force-rebalanced accelerometer. The closed-loop system consists of a MEMS acceleration sensor 100, a forward circuit including a differential capacitive sensing circuit 101 and a loop filter 102, and an electrostatic force feedback circuit 103. The output of the forward circuit is a representative of acceleration.
FIG. 2 shows a block diagram of a Σ-Δ digital feedback force-rebalanced accelerometer, where the forward circuit including a differential capacitive sensing circuit 201, a loop filter 202 as well as a comparator 204 to convert the output analog signal of loop filter 202 to a digital binary stream. The digital binary stream is a representative of acceleration. A feedback circuit 203, in response to the digital binary stream, generates a binary electrostatic restoring force to return the mass to the reference position.
MEMS sensors may be fabricated by surface micromachining or bulk micromachining.
The surface micromachined sensors are fabricated on a single silicon wafer. An example of a surface micromachined accelerometer is shown by U.S. Pat. No. 5,345,824 to Sherman et al in 1994. The surface micromachined accelerometers generally have low sensitivity and high noise, and thus cannot satisfy the requirements of many precision applications.
Bulk micromachined devices use multiple wafers and allow the design and implementation of sensor structures on three-dimensional multiple layer of wafers. A large wafer-thick or double wafer-thick proof mass can be utilized to attain high sensitivity and low noise. Manufacture of bulk micromachined devices require a wafer bonding, which is a complex fabrication step and affects yield as well as the cost of production. Bulk micromachined acceleration sensors are disclosed in prior arts such as U.S. Pat. No. 4,922,756 to Henrion in 1990, U.S. Pat. No. 4,930,043 to Wiegand in 1990, U.S. Pat. No. 5,095,752 to Suzuki et al in 1992, U.S. Pat. No. 5,616,844 to Suzuki et al in 1997, U.S. Pat. No. 5,652,384 to Henrion et al in 1997, U.S. Pat. No. 5,852,242 to Devolk et al. in 1998, U.S. Pat. No. 6,035,694 to Dupuie et al in 2000, U.S. Pat. No. 6,805,008 to Selvakumar et al in 2004, U.S. Pat. No. 6,829,937 to Mahon in 2004, U.S. Pat. No. 7,398,683 to Lehtonen in 2008.
FIG. 3 shows a typical bulk micromachined differential capacitive acceleration sensor in the prior arts, which has a proof mass 302 suspended between two parallel electrode plates 301 and 303 by a spring or multiple springs on a supporting structure. An external acceleration applied on the sensor structure causes proof mass 302 to move and the displacement of proof mass 302 is proportional to the acceleration when the frequencies of the applied acceleration are below a resonant frequency of the sensor structure.
It is known that two parallel electrode plates facing each other form a capacitor which capacitance C is given by:
  C  =            ɛ      ⁢                          ⁢      A              d      0      where ∈ is dielectric constant, A is the facing area of the two electrode plates and d0 is the normal gap between the two electrode plates. The top plate 301 and the top surface of mass 302 form a top capacitor having capacitance CT. And the bottom surface of mass 302 and bottom plate 303 form a bottom capacitor having capacitance CB. Provided that proof mass displacement to a reference position x is zero when proof mass 302 is positioned at the middle and both top and bottom gaps are equal to d0, a positive displacement x of proof mass 302 due to an external acceleration reduces the top gap and increases the bottom gap, and thus changes the capacitances CT and CB differentially. The difference between the capacitances of the top and bottom capacitors is given by
                    C        T            -              C        B              =                                        ɛ            ⁢                                                  ⁢            A                                (                                          d                0                            -              x                        )                          -                              ɛ            ⁢                                                  ⁢            A                                (                                          d                0                            +              x                        )                              =                                    ɛ            ⁢                                                  ⁢            A                                d            0                          ⁢                  (                                    1                              1                -                u                                      -                          1                              1                +                u                                              )                                where      ⁢                          ⁢      u        =                  x                  d          0                    .      It is well known in the arts that the differential capacitance can be detected by a simple sensing circuit such as a switched-capacitor circuit shown in FIG. 4, where the output voltage of the sensing circuit VS is proportional to CT−CB:
      V    S    =                    V        R                    C        F              ⁢          (                        C          T                -                  C          B                    )      
The output voltage of the sensing circuit VS is a linear function of the differential capacitance CT−CB. However, the differential capacitance CT−CB is nonlinear to the proof mass displacement x and therefore, nonlinear to the acceleration sensing. Since CT−CB can also be expressed as:
            C      T        -          C      B        =                    ɛ        ⁢                                  ⁢        A                    d        0              ⁢          (                        2          ⁢          u                +                  2          ⁢                      u            3                          +                  2          ⁢                      u            5                          +        …            ⁢                          )      it can be determined from the expression that the ratio of the first harmonics to its linear term is 1:1. The nonlinearity can be reduced with forcing feedback in a closed-loop system. Such an example is given by U.S. Pat. No. 4,922,756 to Henrion in 1990.
An analog electrostatic forcing can be achieved by applying the output voltage of the forward circuit VO to proof mass 302 while connecting VR to top plate 301 and −VR to bottom plate 303 as shown in FIG. 5. The electrostatic force F on proof mass 302 is given by:
  F  =                    ɛ        ⁢                                  ⁢                  AV          R          2                            2        ⁢                  d          0          2                      ⁡          [                                                  (                              1                -                                                      V                    0                                    /                                      V                    R                                                              )                        2                                (                          1              -                              u                2                                      )                          -                                            (                              1                +                                                      V                    0                                    /                                      V                    R                                                              )                        2                                (                          1              +                              u                2                                      )                              ]      
A Σ-Δ digital electrostatic forcing can be achieved as shown in FIG. 6A and FIG. 6B. As shown in FIG. 6A, while top plate 301 is connected to VR and the bottom plate 303 to −VR, applying −VR to proof mass 302 results forcing up FU, which can be given by:
      F    U    =                    ɛ        ⁢                                  ⁢                              A            ⁡                          (                              2                ⁢                                  V                  R                                            )                                2                            2        ⁢                  d          0          2                      ⁢          1              (                  1          -                      u            2                          )            As shown in FIG. 6B, while top plate 301 is connected to VR and bottom plate 303 to VR, applying VR to proof mass 302 results forcing down FD, which can be given by:
      F    D    =                    ɛ        ⁢                                  ⁢                              A            ⁡                          (                              2                ⁢                                  V                  R                                            )                                2                            2        ⁢                  d          0          2                      ⁢          1              (                  1          +                      u            2                          )            
The capacitive sensing and electrostatic forcing may use the same electrodes alternatively with time multiplexing. An example of such a scheme is given in U.S. Pat. No. 6,035,694 to Dupuie et al. in 2000.
For high-performance accelerometers, a high order Σ-Δ closed-loop system architecture is often required in order to minimize the nonlinearity as well as to lower the quantization noise. Accelerometers of this type for high precision applications are disclosed in prior arts such as U.S. Pat. No. 5,652,384 to Henrion et al in 1997, U.S. Pat. No. 5,852,242 to Devolk et al in 1998, U.S. Pat. No. 6,035,694 to Dupuie et al. in 2000, U.S. Pat. No. 6,805,008 to Selvakumar et al. in 2004. The capacitive accelerometer comprises a MEMS sensor and a signal detection, conditioning and control integrated circuit (IC). The MEMS sensor has a large double wafer-thick proof mass symmetrically suspended between two electrode plates. The accelerometer can be configured as a fifth-order Σ-Δ closed-loop system. The MEMS accelerometer, however, is expensive due to the design and use of four layers of wafers. Besides the wafer usage, the additional bonding process also raises the cost.
Bulk micromachined acceleration sensors with proof mass supported by torsional spring suspension are disclosed in prior arts such as U.S. Pat. No. 6,829,937 to Mahon in 2004 and U.S. Pat. No. 7,398,683 to Lehtonen in 2008. However, the micro structures of this type with short beams to rigidly support large mass on silicon are believed to be prone to impact damage and may not survive in environments of many applications such as seismic data acquisitions and automobile's electronic stability control systems.
It is known in theory and practice that angular accelerations can be measured and calculated with two linear accelerometers rigidly mounted on an object with their sensing axes in parallel. The angular acceleration a of the object's rotation in a plane defined by the two parallel sensing axes separated at a distance D can be determined by
  α  =                    a        2            -              a        1              D  where a1 and a2 are accelerations measured by the two accelerometers respectively. For accelerometers with a given resolution, the precision of the angular acceleration measurement is determined by the distance D which separates the two accelerometers. It is obvious that the distance D cannot be very small or equal to zero. In other words, the two acceleration sensors may not be placed close to each other in a small package for the purpose of measurement of angular accelerations.
The present invention provides a micromachined accelerometer which overcomes one or more limitations of the existing micromachined accelerometers.
It is an object of the present invention to provide a simple and improved acceleration sensor structure for use in closed-loop accelerometer systems with capacitive sensing and electrostatic forcing.
It is another object of the present invention to provide an acceleration sensor structure resilient to shock impact.
It is another object of the present invention to provide an acceleration sensor which is simple and easy to fabricate.
It is another object of the present invention to provide an acceleration sensor which requires a simple signal detection, conditioning and control circuitry for various applications.
It is another object of the present invention to provide an accelerometer of low cost and improved performance.
It is yet another object of the present invention to provide an accelerometer having a pair of acceleration sensors positioned back-to-back, face-to-face or side-by-side for measurement of both linear and angular accelerations.
It is a further object of the present invention to provide an inertial measurement unit which has three pairs of acceleration sensors in back-to-back arrangement and the sensor pairs are further arranged orthogonally from each other for measurement of both linear and angular accelerations of six degrees of freedom of motions.